can i go over the transom rating ?

Discussion in 'Powerboats' started by den44057, Oct 14, 2010.

  1. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    PAR is right. That boat is almost fifty years old. Even if it was lightly used it will still be wise to make sure the transom is up to handling the weight and thrust of a 55 hp. Your boat was rated for 50 but that was 11 years before the USCG sarted regulating HP ratings. The builder may have used the same formula but you can easily calculate it your self. See All you need to know is the centerline length, the width of the transom, the height of the transom, and what kind of steering it uses (remote - a wheel, stick, whatever) or tiller.

    Under USCG regs an owner can put on whatever size engien they want (the HP rating is a manufacturer requirement, not an owner requirement) but some states have made it illegal to exceed the values on the label.

    Anyway many builders down rate HP by 10% for liability reasons so you should not have a problem with 5 more. But I would do a little cautious testing of the boats handling before opening it up. Find out how it runs first, Does it get squirrely? Does it slide a lot in the turns, does it sit really low at the transom? What is the weight difference between a 1961 50 hp and a 1980 55. There could be a big difference. This would make a big differenc in handling. Also in 1961 outboards were rated at the powerhead and a 1980 is rated at the prop, so the engine actually produces a lot more than 55 hp.
  2. tunnels

    tunnels Previous Member

    For sure id would recomend replacing the wood inside the transom with something newer and stronger same thickness just couple inchs deeper.
    Also the ply on the outside replace with piece 1/2 inch aluminium plate as far down as possble and 4 inchs wider than the ply thats already there . Get some monel metal pop rivits with big heads and penny washes and rivit the plate to the back of the boat !!
    Riviting from the outside and washes inside about 3 inchs apart .
    Aluminium has a habit of work hardening with age so need to go outside what you already have there . Got life insurance or a up to date will ? Might need both !!:p
    Love speed and going fast in boats :D!!

    Need to fine tune the motor height as well !!
    The underside of the cav plate on the motor should be level with the bottom of the hull !! Lay a straight edge along and raise the motor a little if need be .and make solid woodpacker to fit in there so it will always stay at that height .
    Makes a world of differance right from day one and the boat will plane and handle better lighter on the steering etc etc !!!
  3. Bglad
    Joined: May 2010
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    Location: Jacksonville, Florida

    Bglad Senior Member

    It doesn't look like there is wood backing lower on the transom so you will need to install something there to catch the lower bolts on the motor to spread the load. Is the transom height right for the 55?
  4. marshmat
    Joined: Apr 2005
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    Location: Ontario

    marshmat Senior Member

    Just a few thoughts, some of which have already been expressed in some form....

    1. Power ratings aren't based on the strength of the transom, they're based on the dimensions and hull shape of the boat. They're supposed to be a reflection of how much power the boat can take without causing handling or stability problems. It's assumed that the builder will make the transom strong enough to handle that power.

    2. The transom structure of a 50 year old boat has certainly deteriorated. A thorough and careful replacement of any rotted wood is likely in order.

    3. A 1980 55 hp is not likely to put out 55 hp today. It would have been within the range 50-60 hp originally (there's a rather wide tolerance in engine labels) but, after 30 years of wear, has likely lost a bit.

    4. Take responsibility for your work. If you're going to do something the manufacturer didn't intend, take the time to do it right, take the time to test it, and take responsibility for whatever happens, good or bad. (On a boat of this age, just about anything falls under 'Mfr didn't intend this' as he likely expected the boat would have been scrapped after twenty or thirty years).
  5. powerabout
    Joined: Nov 2007
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    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    a 1980 55 evi/jono is not rated at the prop it was a few years after that.
    The power rating is also assuming an engine weight and the 1980 55 is a lot lighter than a 55 of early 70's and a lot less torque
    There are many things that create a HP rating, small things like the prop diameter and engine torque and the weight all contribute to the rating.
    The very first V4 evinrude was 50hp in the late 50's that engine will need a much stronger boat than a 55 built in 1980
  6. Bglad
    Joined: May 2010
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    Location: Jacksonville, Florida

    Bglad Senior Member

    I reinforced the transom on a Lowe 1232 similiarly. It was rated for a 3hp but I had a four stroke 4 hp. Fastened an aluminum plate on the exterior to reinforce and raise transom height and backed it up from top to bottom at the interior with plywood. Instead of Monel rivets I used stainless bolts with locking nuts and fender washers at the interior and covered the exterior heads with clear silicon. The transom supports the motor well and the boat scoots along with my son and his friend 12 to 15 mph:)
  7. den44057
    Joined: Oct 2010
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    Location: ohio

    den44057 Junior Member

    ok have been ou all morning so far going over everything on the transom and braces and so far have found nothing cracked or or loose.

    but i did take a better look at the transom plate and it says 15' starcraft jet ?

    could anyone tell me were i can find this aluminum plates to put on the outside of the transome ?

    im about to just sell the 55 hp and buy a 50 hp pretty soon lol now i have two motors the first one was a 66 johnson super sea horse 40 hp that i bought for it and now after all the people saying its a great motor as long as it has no problems i decided not to put it on and am going to sell it.
    Last edited: Oct 17, 2010

  8. den44057
    Joined: Oct 2010
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    Location: ohio

    den44057 Junior Member

    ok guys i had a boat tech over today and he went over the boat and said it can work but would need some aluminum and more wood to be safe.

    but i think im just going to sell the boat i dont want to do all the stuff to it and then just end up buying a little bigger boat anyway.

    so hey if anyone is interested in it make me a offer
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